Novel solvent mixtures

ABSTRACT

CERTAIN MIXTURES OF TETRACHLORODIFLUOROETHANE AND ACETONITRILE ARE USEFUL AS SOLVENTS TO REMOVE ROSIN FLUXES FROM PRINTED CIRCUIT BOARDS. THESE MIXTURES ARE USEFUL BECAUSE OF THEIR UNUSUALLY HIGH SOLVENCY CHARACTERISTICS. A NARROWER CLASS OF SUCH MIXTURES IS PARTICULARLY VALUABLE BECAUSE, IN ADDITION TO HIGH SOLVENCY CHARACTERISTICS, THE MIXTURES EXHIBIT AZEOTROPIC CONSTANT BOILING CHARACTERISTICS, THEREBY FACILITATING HANDLING AND PURIFICATION OF THE SOLVENT MIXTURES WITHOUT SIGNIFICANTLY ALTERING THEIR COMPOSITIONS.

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3,575,867 NOVEL SOLVENT MIXTURES Raymond A. Nesbitt, Morristown, and Francis J. Figiel,

Boonton, N.J., assignors to Allied Chemical Corporation, New York, N.Y. No Drawing. Filed Feb. 6, 1969, Ser. No. 797,245 Int. Cl. (309d 9/00; Clld 7/50; C23g /02 U.S. Cl. 252-171 13 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The electronic industry has sought for solvents which can efliciently remove rosin fluxes from printed circuit boards containing the same. The rosin fluxes are intentionally deposited on the surface of the circuit boards prior to soldering on electronic components, but must be removed after soldering in order to achieve maximum reliability of the printed circuits. The solvent must not only be highly effective for removing the undesired rosin flux but must, for commercial applications, be stable and inert toward the electronic components on the circuit board itself.

A variety of solvents have been tested for such purposes but generally have been found to be lacking, to a greater or lesser extent, one or more of the above-described properties. For example, whereas highly chlorinated solvents, such as CH Cl and CHCI are highly effective for the removal of rosin flux; such solvents, when used alone, attack the electronic components on the circuit board. Such solvents also require the addition of a stabilizer to prevent decomposition. Sym-tctrachlorodifluoroethane (CCl FCCl F) and asymtetrachlorodifiuoroethane (CClF CCl are examples of common solvents which are very stable and which accordingly would not cause any decomposition problems during use. Unfortunately, these reagents exhibit only limited solvencies for rosin fluxes commonly used on electronic assemblies. A variety of non-constant boiling solvent mixtures have been employed to achieve the desired solvency, while retaining the desired inertness toward the electronic components. Such previously known mixtures are not generally known to possess as high a degree of solvency toward rosin fluxes as might be desired. Moreover, preferential evaporation of the more volatile component of such mixtures results in mixtures with changed compositions which may have less desirable properties, such as lower solvency for rosin fluxes.

A number of binary azeotropic (constant boiling) mixtures have been employed for the purpose of cleaning electrical circuits, which afford many of the advantages obtainable with solvent mixtures, but which do not suffer from the above described disadvantage possessed by nonconstant boiling solvent mixtures, Illustrative of such binary azeotropic systems are the azeotrope of 1,1,2-trichloro-l,2,2-trifiuoroethane and methylene chloride, B.P. 37 C./760 mm. (US. Pat. 2,999,817) and the azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane and methyl alcohol, B.P. 39 C./76O mm. (US. Pat. 2,999,816). Unfor- Patented Apr. 20, 1971 tunately, the solvencies of these binary azeotropic compositions for the common rosin fluxes which are employed in the manufacture of printed circuit boards are such that the solvents either attack the boards or components, leave deposits on the boards or become cloudy after use.

It is a major object of this invention to provide novel solvent compositions for rosin fluxes normally encountered on printed circuit boards which novel solvent compositions exhibit a high degree of solvency for such rosin fluxes.

It is another object of this invention to provide novel solvent compositions for rosin fluxes of printed circuit boards which are constant boiling or essentially constant boiling.

Another object of the invention is to provide novel solvent compositions for rosin fluxes used on printed circuit boards which combine the properties of high solvency and inertness to electronic components.

It is a particular object of the invention to provide novel solvent mixtures possessing the stability characteristics of CCl FCCl F and CClF CCl but which exhibit significantly greater solvency properties toward rosin fluxes which are normally found on printed circuit boards.

Other objects and advantages of the invention will be apparent from the following description.

SUMMARY OF THE INVENTION In accordance with the invention, it has been discovered that mixtures consisting essentially of tetrachlorodifluoroethane (C Cl F and acetonitrile (CH CN) in which the weight percent of tetrachlorodifluoroethane is the range of about 7095 exhibit unexpectedly high solvency for rosin fluxes commonly used on printed circuit boards.

Tetrachlorodifiuoroethanes (sym-, CCl FCCl F and asym-, CCIF CCI are commercially produced as mixtures of the two isomers. One commercial process resulting in such mixtures comprises reacting perchloroethylene with HF and chlorine in the presence of an antimony pentahalide catalyst at temperatures in the range of about 30 0 F., followed by distillation. The sym-isomer boils at 925 C. The asym-isomer boils at 91.0 C. The boiling points of the commercial mixtures are intermediate these limits. Since the dilference in boiling points of the two isomers is so small, minor composition changes are of no practical importance and there is no compelling reason to separate the isomers. If desired, however, separation could be efiectively achieved by conventional fractional crystallization procedures.

High purity asym-tetrachlorodifluoroethane can be produced free of its sym-isomer by reacting CH CHF with C1 at 400600 C. followed by simple distillation.

For the purpose of this discussion, the term tetrachlorodifluoroethane or C Cl F will be used in the specification and in the claims to denote sym-tetrachlorodifluoroethane, asym-tetrachlorodifiuoroethane, or mixtures thereof in any proportions.

Compositions as defined within the 70-95 weight percent C Cl F range will dissolve some contaminants which are not soluble in either C Cl F or CH 'CN alone.

Compositions of C Cl F and CHgCN within the indicated weight percent range will remain clear even after repeated use to dissolve rosin fluxes, whereas when either C Cl F or CH CN are used alone, precipitation of the rosin fluxes in the solvents takes place quickly, causing such solvents to become cloudy. Continued use of the solvents in such a state results in redeposition of rosin fluxes on the circuit boards after removal of the solvents.

Further, compositions within the 70-95 weight percent range are inert to electrical components used on printed circuit boards.

It has been further found that azeotropic mixtures are formed at approximately 77 weight percent C CI F and 23 weight percent CHgCN (B.P. 71.5 C./760 mm.) and that these mixtures as well as certain equivalent mixtures in which the weight percent of C Cl F lies between about 4 Example 3 Example 1 is repeated except that pure CClF CCl is used in place of the C Cl F mixture. A constant boiling binary azetrope of CClF CCl and CH CN is formed.

75-85 weight percent, are constant boiling or essentially 5 Example 4 constant boiling. Such mixtures accordingly exhibit little I Qhange in Composition 9 Partial or P The solvency powers of certain mixtures of C Cl F and evaporation such as would occur in normal handling or 1n CHBCN were evaluated by determining their K iusual reclamatmn p 10 Butanol values (K-B values) in accordance with ASTM A preferred class i COIHROSIUOIIS Within the scope of the test D1133-61. In all cases the (3 01 1 component was a invention are those n which the weight percent of the commercial mixture of about 70 Weight percent C2( :1F2 component 1165 between about CCl FCCl F and about 30 Weight percent CClF CCl The P are closest to the true aleotropes 1n Constant results of the evaluations are noted in the following table: bollmg characteristics and are not subject to discernible 15 composition change upon partial or complete evaporation TABLE I or distillation. Still more preferred are the true azeotropic compositions composed of about 77 Weight percent Solvent 5 3? C Cl F When pure CCl FCCL F is used with CH CN, the true azetIope mixture comprises a binary mixture of 1 -3 CCl FCCl F and CH CN. When pure CC1F CCl is used 76.8 weight percent c2014 F2" 6 with =CH CN the true azeotrope comprises a binary mix- 33-3 gig 252232 8 azeQtTOPB-m 121116 Of CC1F2CC13 and CHgCN- When a CCl FCCl F/ 10:0 weight gercent Ol1 l l 1104 CClF OCl mixture is used with CH CN, the true azeofig 8g???- 100.6 trope is actually a mixture of the above-indicated true ssis weight 5mm 020 14131: 58 4 binary ti tt tittfit None of the mixtures within the scope of the invention solo im {vercsnt o Harii'. 1 70 have any adverse efiects upon circuit boards themselves or gf wgffi giggfi 8}???----- 77.8 upon the electronic components affixed thereto. 75.0 weight percent 0101411 I 1 m2 4 The novel mixtures of the invention may be purified and gig reclaimed for use after they have ultimately become 1510 wieght percent OH3CN1: "I- saturated by simple flash distillation. 3%,?) #Ziiit 8%??13111111111 1 105. 0

DETAILED DESCRIPTION AND DISCUSSION 1 Determined by interpolation.

. Examp 1e 1 The above data show that the K-B values of all the A sample of about equimolar amounts of CH CN, B.P. above noted mixtures within the scope of the invention are 82 C., and C CI F (a commercial mixture of about 70 higher than the K-B value of either of the mixture com- Weight percent CCl FCCl F and about 30 weight percent ponen-ts alone. This shows that the solvency powers of such CClF CCl B.P. 91-92.5 (1/760 mm.) is refluxed in a 40 mixtures are greater than those of the mixture components 2,000 ml. pot of a 4' (length) x /2 (diameter) laboratory estill. The temperature at the still head is 71.5 C./760.7 1571341111116 5 mm. This temperature is below the boiling points of either of the mixture components, thereby indicating that an The unexpectedly high y Power Of novel azeotropic system is formed. A sample Of the azeotrope 2 4 2 a ImXtufeS 15 further Shown y the followdistilla-te is analyzed by liquid-gas chromatography and mg data: 1 the presence of CH CN, CCl FCC-l F and CClF CCl is A number of t SHEPS X /2 were wt {T9111 confirmed. The azeotrope is then redistilled but no change standard epoxy f i clmmt ,boards and t coated i in boiling Point or composition is indicated The Cour two common varieties of ros m fluxes. The coated strips position is then determined by calibration of the chromatbaked an oven at 400 for 20 g a then ograms and is found to be: again at 480 F. for 20 seconds. Some of e stnps were Weight percen then completely immersed n a sample of the constant C2C14F2 768 boiling C Cl F /CH CN mixture produced according to CH CN 232 Example 1. Others of the strips were immersed m CH Cl I 3 7 alone and still others of the strips were immersed in Exa l 2 C Cl F (70 weight percent CCl FCCl F/30 weight percent CClF CCl mixture) alone. After seconds im- Example 1 is repeated except that pure CCl FCCl F is mersion in each of the solvents at C., with ultraused in place of the C C1 F mixture. A constant boiling sonic vibration, the strips were removed from the solvents. binary azeotrope of CCl FCCl F and CH CN is formed. 60 The results of the tests were noted in the following table:

APPEARANCE or STRIPS RosIN FLUX Solvent N0 flux Alpha 611 1 London" 77-25-115. 2 CHSCN n but clean Flux gusteredu"- cleafibsome white deposits: O ""1:111:33:1::jjifijffiiiifi fi i" 5235;535:3353??? a ea:

1 "Alpha is a trademark of Alpha Metals, Inc.

2 London is a trademark of London Chemical 00., Inc.

Both of these rosin fluxes are commonly used in the manufacture of printed circuits and are said to contain as major ingredients some form of pine tree gum, a'bietic acid and related substances.

It was observed that each of the used C Cl F and CH CN solvent solutions had turned cloudy. Furthermore, rosin flux agglomerated in the acetonitrile solvent solution upon standing. On the other hand, the used C C1 F /CH CN azeotropic solvent solution was clear and remained so even after standing. The above is evidentiary of the unexpectedly high solvency power of the novel azeotropic composition and equivalents.

The novel solvent mixtures of the invention find other solvent applications such as for removing greases and oils from a variety of industrial items, for the removal of soldering fluxes, for the cleaning of photographic films and prints, for the removal of buffing compounds such as rouge and also may be used as heat exchange media, electrical transfer media, chemical reaction media, hydraulic fluids and as media for a controlled solvation of acrylonitrile-butadiene styrene type resins.

It will be apparent to those skilled in the art that for specialized purposes, various additives could be incorporated with the novel solvent mixtures of the invention, for example, lubricants, detergents and the like. These additives are chosen so as not to adversely affect the essential properties of the mixtures for a given application.

The invention is not intended to be limited by any specific embodiments disclosed herein, but only by the scope of the following claims.

We claim:

1. Mixtures consisting essentially of tetrachlorodifiuoroethane and acetonitrile in which the weight percent of tetrachlorodifluoroethane is in the range of about 70 to 95.

2. Mixtures according to claim 1 in which the weight percent of tetrachlorodifluoroethane is in the range of about 75-85.

3. Mixtures according to claim 1 in which the weight percent of tetrachlorodifluoroethane is in the range of about 80.

4. Mixtures according to claim 1 in which the weight percent of tetrachlorodifluoroethane is about 77.

5. Mixtures according to claim 1 in which the tetrachlorodifluoroethane component is CCl FCCl F.

6. Mixtures according to claim 5 in which the weight percent of CCl FCCl F is in the range of about 75 85.

7. Mixtures according to claim 5 in which the weight percent of CCl FCCl F is about 77.

8. Mixtures according to claim 1 in which the tetrachlorodifluoroethane component is CClF CCl 9. Mixtures according to claim 8 in which the weight percent of CCIFQCCL; is in the range of about 75-85.

10. Mixtures according to claim 8 in which the weight percent of CClF CCl is about 77.

11. Mixtures according to claim 1 in which the tetrachlorodifiuoroethane is a mixture of CCl FCCl F and CCIF CCI 12. Mixtures according to claim 11 in which the weight percent of tetrachlorodifluoroethane is in the range of 7585.

13. Mixtures according to claim 11 in which the weight percent of tetrachlorodifluoroethane is about 77.

References Cited UNITED STATES PATENTS 5/1965 Bauer 252-67 4/1970 Richtzenhain et al. 260-652.5

U.S. Cl. X.R. 

